This invention relates to a spindle orientation control apparatus, and more particularly to a spindle orientation control apparatus which is capable of stopping a spindle and an arbitrary rotational position.
A spindle provided on a numerically controlled machine tool must be capable of rotation at arbitrary speeds. Another capability recently demanded of such machine tools is that the spindle be stoppable at arbitrary rotational positions with a high level of accuracy. Such is the case in machining centers, for example, where a spindle must be stopped at predetermined rotational positions when automatically changing tools, or when carrying out turning, drilling or tapping work on a workpiece at a predetermined angular position thereof by means of a lathe or turning center. In conventional practice, however, the rotational positions at which the spindle is to be stopped are in many cases decided in advance, and it is not possible to stop the spindle accurately at arbitrary rotational positions other than those rotational positions already decided. Although there are some conventional systems that do permit the above (such as disclosed in Japanese Pat. No. 49-47355 published Dec. 14, 1974; Applicant Kerney and Trecker Corporation), these systems require both speed and position control motors for driving the spindle as well as and a motor changeover mechanism such as a clutch, and position and speed control loops must be provided independently of each other. As a result, such systems are high in cost and large in size.